Diagnosis
The humidity sensor is being triggered incorrectly, often because you're using the wrong container or the room is too humid.
How to Fix It
For popcorn, always use a microwave popcorn bag designed for sensor cooking. Place it centered on the flatbed. For 'Melt Butter', use a small, uncovered ceramic or glass dish with the butter in a single layer.
If That Didn't Work
Wipe the interior dry before using sensor cook. Ambient steam or moisture from a previous cook can fool the sensor. Also, never cover the dish with plastic wrap during a sensor function—it traps steam and causes overcooking.
Heads Up
Sensor cooking is an estimate. Start with the minimum time suggested for your food and stop it manually if you hear popping slowing down (for popcorn) or see the butter is melted.